This invention relates generally to gas turbine engines and, more particularly, to the bypass duct and compressor of a turbofan engine.
The starting of aircraft gas turbine engines can be easily accomplished by many different types of on-ground power supply systems. However, for inflight starts, which are necessitated by flame-outs, space and weight considerations make it impractical to carry such power systems aboard the aircraft. When a flame-out occurs in a turbojet engine, there is a large volume of air which passes through the compressor and the resulting windmill speed of the core engine is sufficient to support an inflight start. In the case of a turbofan engine, however, wherein a good portion of the air which enters the inlet of the engine passes around the engine core, the compressor rotor receives a smaller portion of the available ram energy and, therefore, does not attain as high a windmill speed as in the turbojet engine. This is particularly true of a mixed flow-type engine where a common nozzle allows only the duct pressure drop between the core inlet and core exit. If the windmill speed of the core is not sufficient, then an air start cannot be obtained without some kind of a starter assist. Since the ability of an engine to relight varies with altitude and forward speed of the aircraft, a starter assist may not be required over the entire flight envelop of an aircraft, but only a portion thereof, such as, for example, during a low speed flight. Various methods are used for starter assists, such as an auxiliary power unit (APU), cartridge starting, or a combustion air starter. However, the use of any of these starter assist methods is less desirable than making the engine capable of self-starting.
Other characteristics of turbofan engines relates to its operation at ground idle conditions. Because of the high mass flow through the bypass duct, the ground idle thrust is normally greater than is required for the normal taxiing purposes. Further, at these reduced core speeds, the resulting reduced pressure ratios in the compressor tend to bring about some undesirable performance characteristics. For example, at these low pressure ratios, the sump pressurization may not be sufficient to load the carbon seals such that oil leakage may result. Another characteristic is that the customer bleed pressures are reduced at these lower pressure ratios and thus become a limiting factor in determining the minimum idle speed. Another characteristic of these lower speeds is that the compressor discharge pressure and temperature are reduced so as to thereby increase the carbon monoxide emissions.
It is, therefore, and object of the present invention to provide an improved method and apparatus for assisting air starts in a turbofan engine.
Another object of the present invention is the provision in a turbofan engine for an air start assist system which can serve for more than a single purpose of starter assists.
Yet another object of the present invention is the provision in a turbofan engine for an air start assist system which is relatively light in weight, effective in use, and simple in operation.
Yet another object of the present invention is the provision in a turbofan engine for reducing the ground idle thrust level.
Still another object of the present invention is the provision in a turbofan engine for increasing the compressor pressure ratio at low core speeds.
Yet another object of the present invention is the provision in a turbofan engine for reduced bypass flow at idle conditions.
Still another object of the present invention is the provision in a turbofan engine for automatically varying the bypass flow in response to core speed.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.